Vanadium-based catalysts, carried on a siliceous substrate, are usually utilized to oxidize sulphur dioxide to sulphur trioxide; see, for example, British Pat. Nos. 808,639; 898,985; 972,402 and 1,520,336, U.S. Pat. Nos. 3,448,061 and 4,485,190 and European Pat. Nos. 47,540 and 151,823, the content of which is an integral part of the present description. As a siliceous matrix there were used so far a fossil meal (diatomaceous earth) and finely particled silica gel, but it is possible to obtain good results also by using alpha quartz (cristobalite), silicalites, vanadium silicalites or titanium silicalites. For the meaning of these terms, reference should be made to Italian Pat. application No. 22,220 A/87 and to British Pat. No. 2,024,790. These catalysts can be approximately represented by rough formula (I): EQU V.sub.x K.sub.y Na.sub.z O.sub.w S.sub.t (I)
where x, y, w, z and t are broadly varying indexes (depending on the operative conditions and on the ageing degree) and where z can be also equal to zero. Iron (Fe) and other elements (Cs, Al, Mn, Mg etc.) are present too. Still further elements (As, Cl, F, Se etc.), although in very small amounts, are surely poisons for this catalysis. The catalyst, utilizable in both fixed beds and fluid beds, exhibits amounts of vanadyl-alkaline sulphates and pyrosulphates, the concentration of which varies as a function of time. The yields obtainable with these catalysts are high, mainly if use is made of a series of several catalytic layers (at least 3 or 4), of the axial or radial type. Usually, the catalyst is prepared by impregnating a fossil meal or a different siliceous support (average diameter=about 1-40 micrometers), with an aqueous solution of NaOH and of potassium metavanadate (KVO.sub.3) and/or of ammonium metavanadate (NH.sub.4 VO.sub.3), whereafter the meal is thickened, for example by means of carboxy-methylcellulose or a polyacrylamide. There follows an extrusion, whereby differently shaped particles (solid, hollow or polylobed cylinders, optionally having helical grooves, etc.) are formed. Prior to use, the catalyst is activated in a SO.sub.2 flow and a final activation is operated by the same process fluid, containing SO.sub.2, SO.sub.3, O.sub.2 and N.sub.2, at the reaction temperature (350.degree.-650.degree. C.) During the oxidation of SO.sub.2 to SO.sub.3 the activated catalyst material is in the form of a liquid film on the support pores surfaces. The average life of these catalysts ranges from 5 to 10 years with a progressive decrease of the vanadium content (for example from about 7% to about 5% by weight). The possibility of regenerating and re-using the exhausted catalyst would represent an advantage of great industrial importance. However, it was not possible so far to carry into effect any of the various supposed alternatives.
According to a first alternative [see Journal of Catalysis 43, 243-251 (1976)], the exhausted catalyst should be attacked by a strong acid (HCl), capable of solubilizing the vanadium, and the same vanadium should be recovered by extraction. This method, however, was not practised due to the presence of an insoluble siliceous gangue which effectively retains a not negligible vanadium amount. The difficulties encountered in the separation of the gangue could not be overcome so far, at least from an industrial viewpoint.
A second alternative (see again said article in Journal of Catalysis) comprises heating the exhausted supported catalyst to very high temperatures (500.degree. C.), in order to remove arsenic and the other poisons, and a subsequent grinding of the calcination residue. It is then necessary to knead the ground residue with sulphur and with an ammonium sulphate solution, whereafter an activation with air at 750.degree.-800.degree. C. is carried out. Neither this method, however, is free from drawbacks. Compression strength, ageing resistance and abrasion resistance are not fully satisfactory. Furthermore, the SO.sub.2 conversion yields often sink to not acceptable values.
A third alternative, at last, contemplates a possible recovery from an exhausted catalyst having a silica (SiO.sub.2) content higher than 80% by weight. In this case, however, as a consequence of a too long exploitation of the catalyst, a non-negligible amount of the catalytic activity gets irremediably lost.
The Applicant has now found that it is possible to prepare an excellent catalyst (free from said drawbacks), in an utmost rapid and simple manner, by starting from an already exploited and now exhausted catalyst.